This invention relates to a system of numerical control unit, and more particularly to a novel and flexible control system based on a programmed control system.
Numerical control techniques are applied to various industrial machines, e.g., lathes, milling machines, machining centers, punching presses, grinding machines, drafters, wiring machines, and gas cutting appratuses. The numerical control units for the industrial machines are required to take a wide variety of different forms, because the functional requirements of the machines to which such units are applied vary vastly; namely, presence or absence of contouring function, presence or absence of feed rate control function in synchronism with the rotation of the main shaft, different specifications for functions of input medium of control information, or the like. For each of such different functional requirements, one logic means is prepared by using logical operational means. Accordingly, the conventional numerical control unit has a critical shortcoming in that a different numerical control unit is necessary for each different machine, so that the design and manufacture of the unit are very complicated, and its production cost is high.
To avoid such shortcoming, a computer numerical control (CNC) has been developed, in which a mini-computer is included for fulfilling the functions of a numerical control unit by the operational functions of the minicomputer. The conventional CNC system, however, has shortcomings in that the mini-computer itself is comparatively expensive, that complicated electric circuit means is necessary for transferring signals between the minicomputer and the external machine tool, and that complicated programs are required. More particularly, the minicomputer is generally organized on the basis of 16 bits per word, and it can treat at most 2.sup.16 bits at a time, while a typical numerical control unit requires an ability of treating more bits at a time (for instance, in the case of a numerical control operation involving a maximum instruction value of 8 meters with a pulse unit of 0.001 millimeter, 2.sup.24 bits should be treated at a time). Thus, the minicomputer in a numerical control system uses complicated programs, e.g., for the operation of interpolation a process involving two divided words is used. As a result, the mini-computer must have a core memory of 8 to 16 KW, and it becomes very expensive.
The numerical control unit is generally required to display the present position of the table of a machine tool being controlled by a suitable display means, such as numeral display tubes. The value of such present position is generally stored in the main memory of the mini computer. In order to extract and display the present positional values of each operating shaft of a machine tool, which values are ever changing at a high speed, interruption or other similar techniques must be used. This causes another shortcoming in that the interruption or the like further complicates the computer circuitly and that operating time for main functions is so interfered with by the interruption or the like that adequate interpolating operations become difficult.